| Polymer electrolyte fuel cells?PEFCs?are thought to be superior power sources for electric vehicles thanks to their advantages,such as low-temperature working condition,quick start-up,reduced corrosion and convenient electrolyte management.Common proton exchange membrane fuel cells?PEMFCs?use perfluorosulfonic acid membrane Nafion?as polymer electrolyte and Pt as catalyst,which cost too much and limit their large-scale commercialization.Non-precious metal such as Co and Ni can serve as catalyst in anion exchange membrane fuel cells?AEMFCs?with enhanced efficiency of oxygen reduction reaction.Over the last decade,AEMFCs have recieved more and more attention and developed rapidly.However,comparing with the state-of-the-art Nafion?,there are some barriers such as lower ionic conductivity,weaker mechanical properties and worse chemical stability in anion exchange membranes,which need to be overcome to fulfill the requirements of fuel cell application.In this paper,novel poly?arylene ether sulfone?s containing symmetric side-chain benzyltrimethyl ammonium groups were designed and synthesized,with the purpose of studying the distribution of side-chain ammonium groups and the steric effect of the hydrophobic benzyl groups on the water channels.Detailed contents are as follows:1.A difluorodiphenyl sulfone monomer containing four pendant benzyl groups,namely3,3',5,5'-tetra?4”-methylphenyl?-4,4'-difluorodiphenylsulfone?4C-DFDPS?,was synthesized via Suzuki coupling reaction in high yield.By controlling the molar ratio of 4C-DFDPS and difluorodiphenylsulfone?DFDPS?,different copoly?arylene ether sulfone?s were synthesized via nucleophilic substituting condensation with bisphenol A.The subsequent bromination,quaternization and alkalization of the copolymers gave 4C-y-Q anion exchange membranes containing four symmetric benzyltrimethyl ammonium groups.The QTBM-y membranes containing four asymmetric benzyltrimethyl groups,which has been reported before,were introduced to compare with the 4C-y-Q membranes.4C-y-Q membranes were investigated in terms of water uptakes,swelling ratios,ionic conductivities,ion exchange capacities,?,mechanical properties,thermal stabilities as well as long-term chemical stabilities.The results suggest that symmetric side-chain type anion exchange membranes are more ionic conductive with better defined microphase seperation structure,while the asymmetric side-chain ones are more dimensionally stable.2.3,3',5,5'-tetra?3”,5”-dimethylphenyl?-4,4'-difluorodiphenyl sulfone?8C-DFDPS?was synthesized with similar procedure.Copolymers 8C-y with different contents of pendent benzyl groups were synthesized by adjusting the molar ratio of 8C-DFDPS and DFDPS,which reacted with bisphenol A via nucleophilic substituting polymerization,.8C-y-Q membranes containing hydrophobic benzyl side chains were obtained by subsequent bromination,quaternization and alkalization.The results of transmission electron microscopy indicated that the water channels in 4C-y-Q membranes were more continuous and the ion clusters were larger.Ionic conduction efficiency was enhanced thanks to a better defined microphase separation in 4C-y-Q membranes.The 4C-60-Q membrane showed higher ionic conductivity(206 mS cm-1 at 80°C)than most of the reported membranes.Ionic conduction efficiency??/IEC?could be improved and made higher than that of Nafion?with lower ion transport activation energy(6.61 kJ mol-1).The Poly?arylene ether sulfone?s backone could be stablized to some degree by introducing methyl groups on the side-chain phenyl groups.Benzyltrimethyl ammonium groups were more stable in the alkaline condition without the methyl groups on the benzene rings. |
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